1. Field of the Invention
The present invention relates to a method for manufacturing a printed wiring board by mean of laser via machining as well as to a copper-clad laminate and treatment solutions used therefor.
2. Description of Related Art
With miniaturization and lightening of electronic devices, a high density printed wiring boards is being required and highly multi-layered printed wiring boards are being developed lately. It is then necessary to assure conduction between wiring layers on which circuits are formed through an intermediary of an insulating layer in implementing the multi-layered printed wiring board and hence inter-layer connection has become an important element in the technology for manufacturing such multi-layered printed wiring board.
As types of inter-layer connection of wiring layers, there is a through hole as well as a blind via hole and an interstitial via hole, i.e., non-through holes. While methods for forming such holes include a drill machining method, a laser machining method and others, the laser machining method has become the main stream from the aspects of downsizing of the machined hole and of high machining speed. Above all, CO2 lasers having high laser energy have spread most. However, it has been difficult to machine such holes because laser light is reflected on a surface of copper foil in a wavelength range of the CO2 laser. Then, a large window method of machining holes after etching and removing only copper foil around a hole to be formed in advance has been adopted.
However, because the large window method requires a step of patterning the copper foil as well as a copper foil etching diameter that is twice the size of a diameter of laser for correction of misalignment of laser machined hole, it has been difficult to design fine wiring. Then, there has been discussed a direct via machining technology of directly machining copper foil and an insulating layer by laser in the same time. The direct via machining method allows machining of the copper foil by roughening the surface of the copper foil to increase its laser light absorptance. The process for roughening the surface of the copper foil includes a blackening treatment of forming a copper oxide film of needle-like crystal, a treatment of directly roughening copper by grain-boundary etching of copper, and others.
The following are documents related to such technologies:    Japanese Patent Laid-Open No. 2002-217551,    Japanese Patent Laid-Open No. 2004-6611, and    Toshiki Hirogaki and four others, “Improvement of Quality of Hole in Cu Direct Machining by CO2 Laser”, the 14th Micro-Electronics Symposium, October, 2004
When the direct via machining is carried out by using laser through a copper-clad laminate in which a roughening treatment has been implemented on the surface of copper foil, molten and scattered Cu that is caused by Cu melted by heat of the laser and that scatters and sticks convexly around a via opening is generated and a so-called overhang by which a bore of the outer-layer copper foil becomes smaller than a bore of an insulating layer is generated. FIGS. 1a and 1b are schematic drawings showing the molten and scattered Cu and overhang generated by performing the direct via machining through the copper-clad laminate, wherein FIG. 1a is a plan view (top view) and FIG. 1b is a section view along a line A-A in FIG. 1a. When laser light is irradiated to machine a via through the copper-clad laminate on which an outer-layer copper foil 3 is formed on an insulating layer 1 having an inner-layer wiring 2 and a copper oxide film 4 is formed on the surface thereof, the molten and scattered Cu and overhang 5 are generated around the opening. The molten and scattered Cu and overhang 5 must be removed because they lower adhesion of plating around the via and flatness in lamination in later steps. They may be removed by physical grinding by means of a buff or by chemical etching by means of a chemical agent. However, there has been problems that the outer-layer copper foil or inner-layer wiring is ground or etched, thus reducing a wiring thickness, by the treatment for removing the molten and scattered Cu and overhang.
Accordingly, it is an object of the invention to provide a direct via machining process as well as a copper-clad laminate and treatment solutions used therefor that allow the molten and scattered Cu and overhang to be removed without reducing the wiring thickness.